Electronic imaging systems that include light valves for spatially modulating a light beam with information are well known in the prior art. Common applications of such systems include projection displays and printing systems. Typically, these systems have taken the basic form of a white light source, most notably an incandescent or arc lamp, illuminating one or more light valves or spatial light modulators with appropriate color filtering to form the desired image, the image being projected onto a viewing screen or photosensitive medium.
Lasers have been known to be attractive alternative light sources to lamps for light-valve-based projection displays and printing systems. One potential advantage of lasers is a wider color gamut featuring very saturated colors. Laser illumination offers the potential for simple, low-cost efficient optical systems, providing improved efficiency and higher contrast when paired with some conventional light valves. One disadvantage of lasers for projection display is the lack of a cost-effective laser source with sufficient power at appropriate visible wavelengths.
The lack of availability of low-cost visible lasers suitable for electronic imaging systems in displays and photographic printers has largely hampered the development of light-valve-based laser electronic imaging systems for these markets. In a typical television application, for example, red, green, and blue lasers are required at a power level of approximately 1 W each. Currently available blue and green lasers at that power level, such as intracavity-doubled diode-pumped solid-state lasers, are extremely expensive due to the need for diode laser pumping of a solid-state laser crystal, assembly of a resonator, and the need for nonlinear frequency conversion to produce visible light. There are further problems with stability and lifetime that must be addressed. Furthermore, single lasers for each color are in many ways undesirable for a display or printing system. Because these lasers are both spectrally and spatially coherent, coherence artifacts within the optical systems, as would arise from etalon effects in air gaps or thin glass elements in the system, should, preferably, be avoided. In display systems, speckle arising due to the rough display screen has to be defeated, which is made difficult by the spectral and spatial coherence of the laser sources. Finally, as has already been discussed, the area light valves of interest do not require a very spatially coherent source. Hence, it would be desirable to use a laser source with low spectral and spatial coherence.
Light valves that consist of a two-dimensional array of individually operable pixels, arrayed in a rectangle, provide another component that enable laser display and printing systems. Examples of area light valves include reflective liquid crystal modulators, such as the liquid-crystal-on-silicon (LCOS) modulators that are available from JVC, Three-Five, Aurora, and Philip; and micromirror arrays, such as the Digital Light Processing (DLP) chips available from Texas Instruments. Advantages of two-dimensional modulators over one-dimensional array modulators and raster-scanned systems are the absence of required scanning; absence of streak artifacts due to nonuniformities in the modulator array; and immunity to laser noise at frequencies much greater than the frame refresh rate (≧120 Hz) in display systems. A further advantage of two-dimensional spatial light modulators is the tolerance for low spatial coherence of the illuminating beam. In contrast, one-dimensional or linear light valves, such as the Grating Light Valve (GLV) produced by Silicon Light Machines and conformal grating modulators, require a spatially coherent illumination in the short dimension of the light valve.
One means of providing laser light with low spatial and spectral coherence is to utilize multiple laser sources. International Application Published Under the Patent Cooperation Treaty (PCT), International Publication No. WO 95/20811, published on Aug. 3, 1995 by Waarts et al., titled “Laser Illuminated Display System” discloses the use of multiple diode lasers multiplexed and fiber-coupled to illuminate a spatial light modulator. U.S. Pat. No. 6,318,863 issued Nov. 20, 2001 to Tiao et al., and titled “Illumination Device And Image Projection Apparatus Including The Same,” discloses an illumination device using multiple light sources (laser diodes are used in one embodiment) coupled to an array of tapered light pipes to illuminate an area light valve. Other instances of the prior art have used laser arrays. U.S. Pat. No. 5,704,700 issued Jan. 6, 1998 to Kappel et al., and titled “Laser Illuminated Image Projection System And Method Of Using Same,” discloses an image projection system in which a microlaser array is coupled to a beam shaper to illuminate a light valve. Furthermore, U.S. Pat. No. 5,923,475 issued Jul. 13, 1999 to Kurtz et al., and titled “Laser Printer Using A Fly's Eye Integrator,” discloses a printing system using a diode laser array and a light valve.
When using an area light valve in a display or printing system requiring the use of RGB laser arrays, one often desires to use fully integrated two-dimensional laser arrays. One of the few laser technologies that are easily integrable in two dimensions is a vertical-cavity surface-emitting laser (VCSEL).
VCSELs, based on inorganic semiconductors, (e.g., AlGaAs), have been developed since the mid-80's (“Circular Buried Heterostructure (CBH) GaAlAs/GaAs surface Emitting Lasers” by Susumu Kinoshita et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June 1987). They have reached the point where AlGaAs-based VCSELs, emitting at 850 nm, are manufactured by a number of companies and have lifetimes beyond 100 years (“Vertical-Cavity Surface Emitting Lasers: Moving from Research to Manufacturing” by Kent D. Choquette et al., Proceedings of the IEEE, Vol. 85, No. 11, November 1997). With the success of these near-infrared lasers, attention in recent years has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (“Vertical-Cavity Surface-Emitting Lasers” by Carl W. Wilmsen et al., Cambridge University Press, Cambridge, 2001). There are many potential applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (“2.5 Gbit/s 100 m data transmission using graded-index polymer optical fibre and high-speed laser diode at 650 nm wavelength” by T. Ishigure et al., Electronics Letters, Mar. 16, 1995, Vol. 31, No. 6). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) that produce light output spanning the visible spectrum.
In an effort to produce visible wavelength VCSELs it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy a number of advantages over inorganic-based gain materials in the visible spectrum. For example, typical organic-based gain materials have the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, emit over the entire visible range; can be scaled to arbitrary size; and most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip. Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The laser gain material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as VCSEL (see U.S. Pat. No. 6,160,828 issued Dec. 12, 2000 to Kozlov et al., and titled “Organic Vertical-Cavity Surface-Emitting Laser”), waveguide, ring microlasers, and distributed feedback (see also, for instance, “Organic solid-state lasers” by G. Kranzelbinder et al., Rep. Prog. Phys. 63, 2000, and U.S. Pat. No. 5,881,083 issued Mar. 9, 1999 to Diaz-Garcia et al., and titled “Conjugated Polymers As Materials For Solid State Laser”). A problem with all of these structures is that in order to achieve lasing it is necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities, since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically-pumped organic lasers is the low carrier mobility of organic material, which is typically on the order of 10−5 cm2/(V−s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (“Study of lasing action based on Förster energy transfer in optically pumped organic semiconductor thin films” by V. G. Kozlov et al., Journal of Applied Physics, Vol. 84, No. 8, Oct. 15, 1998). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales proportional to the carrier mobility), low carrier mobilities result in orders of magnitude having more charge carriers than singlet excitons. Consequently, charge-induced (polaron) absorption can become a significant loss mechanism (“Pulsed excitation of low-mobility light-emitting diodes: Implication for organic lasers” by N. Tessler et al., Applied Physics Letters, Vol. 74, No. 19, May 10, 1999). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (“Light amplification in organic thin films using cascade energy transfer” by M. Berggren et al., Letters to Nature, Vol. 389, Oct. 2, 1997), and ignoring the above mentioned loss mechanisms, puts a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms places the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, that can be supported by organic devices (“High Peak Brightness Polymer Light-Emitting Diodes” by Nir Tessler, et al., Advanced Materials, 1998, 10, No. 1).
One way to avoid these difficulties is to use crystalline organic material instead of amorphous organic material as the lasing media. This approach was recently taken (“An Organic Solid State Injection Laser” by J. H. Schon, Science, Vol. 289, Jul. 28, 2000) where a Fabry-Perot resonator was constructed using single crystal tetracene as the gain material. By using crystalline tetracene, larger current densities can be obtained; thicker layers can be employed (since the carrier mobilities are on the order of 2 cm2/(V−s)); and polaron absorption is much lower. This organic structure results in room temperature laser threshold current densities of approximately 1500 A/cm2.
One of the advantages of organic-based lasers is that since the gain material is typically amorphous, devices can be formed inexpensively when compared to lasers with gain materials that require a high degree of crystallinity (either inorganic or organic materials). Additionally, lasers based upon organic amorphous gain materials can be fabricated over large areas without regard to producing large regions of single crystalline material; as a result they can be scaled to arbitrary size resulting in greater output powers. Because of their amorphous nature, organic-based lasers can be grown on a wide variety of substrates; thus, materials such as glass, flexible plastics, and silicon are possible supports for these devices. Thus, there can be significant cost advantages as well as a greater choice in usable support materials for amorphous organic-based lasers.
An alternative to electrical pumping for organic lasers is optical pumping by incoherent light sources, such as light emitting diodes (LEDs), either inorganic (“Semiconducting polymer distributed feedback laser” by M. D. McGehee et al., Applied Physics Letters, Vol. 72, No. 13, Mar. 30, 1998) or organic (U.S. Pat. No. 5,881,089 issued Mar. 9, 1999 to Berggren et al, and titled “Article Comprising An Organic Laser”). This possibility is the result of unpumped organic laser systems having greatly reduced combined scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. Even taking advantage of these small losses, the smallest reported optically-pumped threshold for organic lasers to date is 100 W/cm2 based on a waveguide laser design (“Light amplification in organic thin films using cascade energy transfer” by M. Berggren et al., Nature, Vol. 389, Oct. 2, 1997). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, it is necessary to take a different route to incorporate optical pumping by incoherent sources. What is needed is a method of minimizing gain volume in a laser area structure while enabling optically pumped power density thresholds below 5 W/cm2.